Prior art furnaces for cracking of hydrocarbons have tubes made from nickle base alloys with a relatively high chromium content. This composition brings with it several disadvantages, including that the tube material is expensive and does not have a fully satisfying resistance to carburization and formation of carbides, primarily chromium carbide. Further the shape durability (dimensional stability) of these tubes which are designated as being formed of a high temperature material, is not fully sufficient in some applications.
A cracker is used for the cracking of hydrocarbons. The starting material can be e.g. naphtha or propane mixed with a smaller amount of steam. When the gases are passed through the tubes in the cracking furnace, its temperature is increased up to about 850.degree. C. Among important products which are obtained are ethylene and propene. Further hydrogen, methane, butene and other hydrocarbons are obtained. In order to avoid unwanted reactions, it is essential that the heating be very rapid and that the products which are obtained are thereafter rapidly cooled. The residence time in the furnace is only a few tenth of a second. The temperature in the furnace is 1100.degree.-1200.degree. C. and the temperature of the gases in the tubes in the furnace can be more than 1100.degree. C. Heating of the furnace can be performed by burning gases from the cracking process, e.g. hydrogen and methane and a furnace may be equipped with a great number of burners, which can be positioned in the bottom and sides of the furnace.
The tubes which are used in the furnace should have the ability to withstand the high temperatures with a good shape durability or dimensional stability. They must also be resistant against oxidation and corrosion in order to tolerate the atmosphere present in the furnace. The carbon potential inside the tubes in the furnace is very high and the tube material should therefore be resistant against carburization and formation of carbides. Small amounts of sulphur are often added to the starting materials and the tubes must thus also be resistant to sulphur and sulphur compounds. On the inside of the tubes, deposits of carbon and coke form which may cause local temperature variations. These deposits may be removed suitably by oxidation thereof with steam.